1. Field of the invention
The present invention relates to a toner composition for use in a developer for the development of electrostatic latent images, and more particularly, the invention relates to a toner composition with excellent charge stability, flowability, feedability to the developing unit, and cleanability from the photosensitive means. The present invention also relates to a method for preparing the toner composition.
2. Description of the prior art
Previously, in copying machines utilizing electrophotography, an electrostatic latent image is formed on the photosensitive means which has a photosensitive layer containing inorganic or organic photoconductive materials, and various types of powder toners comprising a resinous binder and additives such as colorants dispersed therein have been used in order to visualize the electrostatic latent image by dry development.
In the above-mentioned electrophotographic processes, the electrostatic latent image formed on the photosensitive means by the sequence of charge and exposure is developed with the powder toner, thereby forming a toner image corresponding to the electrostatic latent image, and this toner image is then transferred to a support such as transfer paper, and finally the toner image is fixed onto the support by use of a fixing means such as a heating or pressurizing roller, thus obtaining the desired copy. After the toner image has been transferred to the support, the surface of the photosensitive means is cleaned by scraping with a cleaning blade in order to remove the residual toner.
In order to achieve satisfactory image formation in the above type of system, the characteristics of the toner must satisfy various requirements with respect to every phase of the imaging process, that is, stable charge retention, maintenance of superior development characteristics such as the absence of fogging or aerial scattering, no adhesion of residual toner on the surface of the photosensitive means in the cleaning process, etc.
Accordingly, in recent years, in Japanese Laid-Open Patent Publication No. 60-186851 and elsewhere, techniques have been proposed for improving charge stability and cleaning characteristics by use of a toner composition containing acrylic polymers in finely powdered form.
However, the toner composition proposed in the Japanese Laid-Open Patent Publication No. 60-186851, consisting merely of toner particles with an externally added fine acrylic polymer powder, still fails to have a satisfactory flowability. In particular, when applied to the continuous or high speed duplication of a large number of frames under high humidity conditions, the decrease in flowability causes toner scattering or image fogging in some cases.
Moreover, the developing units employed in recent types of copying machines are generally so constructed that toner replenished from a toner cartridge is accommodated in a container called a hopper, a feed roller composed of a porous or elastic material such as sponge is fitted in the bottom of the hopper, and the rotation of this feed roller allows toner to drop into the developer mixing unit to replenish the amount consumed, whereupon the toner is charged again and fed to the developing sleeve. However, if the proposed toner composition mentioned above is applied to this type of process, because of poor flowability, the toner composition may not drop despite the rotation of the feed roller. Moreover, this drawback becomes extremely pronounced under high humidity conditions.